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Minor: Add comment explaining rationale for hash check #11750

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Aug 1, 2024
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Minor: Add comment explaining rationale for hash check #11750

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10 changes: 7 additions & 3 deletions datafusion/physical-plan/src/aggregates/group_values/row.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -122,10 +122,14 @@ impl GroupValues for GroupValuesRows {

for (row, &target_hash) in batch_hashes.iter().enumerate() {
let entry = self.map.get_mut(target_hash, |(exist_hash, group_idx)| {
// verify that a group that we are inserting with hash is
// actually the same key value as the group in
// existing_idx (aka group_values @ row)
// Somewhat surprisingly, this closure can be called even if the
// hash doesn't match, so check the hash first with an integer
// comparison first avoid the more expensive comparison with
// group value. https://github.com/apache/datafusion/pull/11718
target_hash == *exist_hash
// verify that the group that we are inserting with hash is
// actually the same key value as the group in
// existing_idx (aka group_values @ row)
&& group_rows.row(row) == group_values.row(*group_idx)
});

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